It is known to use various different types of apparatus to monitor usage of a road, which is done for various purposes, including to identify congested areas and plan future infrastructure.
Currently-used apparatus includes various types of overhead sensors, for example laser sensors or video cameras. However, these types of sensors suffer from reduced accuracy whenever there is rain, mist or snow. They can be expensive to install, and their overhead location makes them vulnerable to damage by electrical storms and sometimes vandalism.
Inductive loops are also known, and can be buried in the road to detect traffic as it passes over the loop. Inductive loops are generally cheaper to install, more reliable and less vulnerable to damage as compared with overhead sensors, but suffer from a lack of accuracy in certain road conditions. For example, in an area where vehicles often change lanes, vehicles can be mis-counted if they are straddling two lanes when passing over the loops. This is because it can be difficult to determine whether inductive disturbances occurring in loops in two adjacent lanes result from passage of two vehicles in adjacent lanes, or from a single vehicle straddling the two lanes. Motorcycles are also prone to mis-counting, since they often drive between lanes, and sometimes two motorcycles ride side-by-side in a single lane.
EP1028404 discloses an arrangement of two inductive loops, one after the other along each lane. The loops are each positioned substantially centrally of each lane, and the lateral gap between the edges of the loops is around 1.5 meters (5 feet) in a typical installation. This system works well for traffic which has good lane discipline, but accuracy suffers in situations where many vehicles are changing lanes over the measurement site, or where the vehicle mix includes more than a small proportion of motorcycles. It is sometimes possible to distinguish one straddling vehicle from two in-lane vehicles by evaluating the geometric mean of the peak change in inductance in loops in adjacent lanes, and testing the geometric mean against a threshold, but the accuracy of this method leaves room for improvement, and motorcycles are often missed altogether.
It is an object of the invention to reduce or substantially obviate the above mentioned problems.